A terminal for use in a connector is normally subjected to surface treatment for corrosion resistance and endurance. As the surface treatment, first, substrate plating is applied, and upper layer plating is further applied thereon. The upper layer plating is selected accordingly based on the use application at a terminal portion. For example, for use at a contact portion with an object, gold-containing material plating, silver plating, tin-containing material plating and the like are cited considering connection stability and cost. Moreover, for use at a connecting portion with a board and the like, gold-containing material plating, tin-containing material plating and the like are cited considering soldering property, environmental issue and cost.
First, patent documents 1, 2, 3 and 4 will be presented as documents of suppressing the occurrence of whiskers.
Next, patent documents 5, 6, 7, 8 and 9 will be presented as of multilayer surface treatment.
Patent Document 1: Japanese Patent Application Laid-open No. 2006-127,939
According to the summary of patent document 1, it is disclosed that in an electric conductor component such as a lead-free tin plated flat cable, with an object of providing an electric conductor component preventing the occurrence of whiskers at a portion receiving external stress and a production process of the same, tin plating 2 with a thickness of from 0.2 μm to less than 1.0 μm is applied to an electric connecting portion, the tin plating is heat treated to have a ratio of an alloy layer 4 of tin of the tin plating 2 and an electric conductor 1 of not less than 50%, not less than 1.0% of bismuth is also added to the tin plating 2 to enhance solder wettability, nickel plating 5 with a thickness of 0.1 μm to 2.0 μm as a substrate metal is applied to prevent oxidation degradation of the electric conductor, and further a sealing agent is applied to the tin plating 2 to enhance the reliability of electric connection.
Patent Document 2: Japanese Patent Application Laid-open No. 2004-292,944
According to the summary of patent document 2, it is disclosed that with an object of providing a metal material of an electric component for semiconductor preventing the occurrence of whiskers, when an Sn plating layer is provided on a surface of the metal material of an electric component, 0.5 to 5 μm of Ni is plated and 0.5 to 5 μm of Cu is further plated thereon as a substrate.
Patent Document 3: Japanese Patent Application Laid-open No. 2007-53,039
According to the summary of patent document 3, it is disclosed that with an object of providing a connecting structure of an electric connector capable of suppressing whiskers occurred due to external stress added to a connecting portion by applying tin-based plating containing silver to a contact portion where a connector and a flexible wiring board are bonded, a connecting structure of an electric connector for a flexible wiring board 1 comprises a connecting hole for inserting a connecting portion of the flexible wiring board 1 therein, a plurality of metal terminals 8 loaded in a terminal loading space formed toward inside of a housing 7 of an electric connector 6 from the connecting hole, a separating wall for electrically separating between the metal terminals so that the adjacent metal terminals 8 do not contact, and a conductive layer 4 portion where metal terminals of the electric connector 6 are pressure contacted with the connecting portion of the flexible wiring board 1, and a tin layer containing silver with a predetermined thickness is formed on a copper pattern with a predetermined thickness in the conductive layer 4 portion.
Patent Document 4: Japanese Patent Application Laid-open No. 2007-103,586
According to the summary of patent document 4, it is disclosed that with an object of providing a process for producing a wiring circuit board capable of suppressing the occurrence of whiskers, ensuring connectivity with an electric component and reducing variation of the connectivity, a wiring pattern 12 of a metal thin film 31 and a conductor layer 33 is formed on a base insulating layer BIL, an electroless tin plating layer 34 is formed so as to cover the wiring pattern 12, and the wiring pattern 12 and the tin plating layer 34 are subjected to heat treatment here. A mixed layer 35 of copper and tin is formed by the heat treatment with a heat treating temperature of 175 to 225° C. and a heat treating time of 2 to 10 minutes, thereafter a solder resist SOL is formed so as to cover the wiring pattern 12 and the tin plating layer 34 in a predetermined area on the base insulating layer, and then the solder resist SOL is subjected to thermal curing treatment.
Patent Document 5: Japanese Patent Application Laid-open No. 2006-161,155
According to the summary of patent document 5, it is disclosed that when a 4 layer plated film of zinc, copper, nickel and aluminum is formed on a magnesium alloy and the configuration of the plated matter is complexified, a film having 3 layers of nickel, copper and aluminum in order from magnesium alloy side is formed on the magnesium alloy by a plating method and further a portion of the aluminum layer on the surface is anodized to solve the problem that the adhesion of the entire film decreases after aluminum plating due to internal stress of nickel plated film. By forming the copper plated film between the nickel plated film and the aluminum plated film as a stress relaxation layer, the internal stress generated in the nickel plated film and the aluminum plated film is relaxed and the adhesion of the entire film is enhanced.
Patent Document 6: Japanese Patent Application Laid-open No. 2006-295,114
According to the summary of patent document 6, it is disclosed that with an object of providing a board for an electronic device capable of pulling a core board away from the board for an electronic device side with weak force, reducing load of a chemical or electrochemical solution method or mechanical abrasive operation for exposing an electrode plane to a lower plane, and further downsizing, a production process of the same, the electronic device and a production process of the same, a photo solder resist (PSR) film 102 where an electric component is loaded is formed on a core board 101 of metal, a plurality of metal electrodes 110 of 3 layers of a gold plated film 104, a nickel plated film 105 and a gold plated film 106 is formed at a predetermined position inside of the PSR film 102 so as to penetrate the PSR film 102 in a thickness direction, and the core board 101 is chemically dissolved and removed by injection of ferric chloride solution at the final step of producing an electronic device.
Patent Document 7: Japanese Patent Application Laid-open No. 2007-9,305
According to the summary of patent document 7, in a terminal of 3 layer plated film of an electroless nickel plated film, an electroless palladium plated film and an immersion gold plated film, a gold plated film is formed by only immersion gold plating which is easy to use to provide an electroless palladium plating solution having an excellent bonding strength in both wire bonding and solder ball and also being excellent in running property even when an electroless nickel-phosphorous with high versatility is used, and a problem is solved by an electroless palladium plating solution containing at least components (a) a palladium salt organic complex, (b) a monocarbonic acid having a sulfide group or its salt and (c) a hypophosphorous acid or its salt.
Patent Document 8: Japanese Patent Application Laid-open No. 2007-88,211
According to the summary of patent document 8, it is disclosed that with an object of providing an implementable lead frame having wire bondability and using lead-free solder, wherein the reliability of the semiconductor device is maintained without deteriorating the adhesion between the lead frame and resin for resin sealing of the semiconductor device and the adhesion is improved without receding roughening effect in a surface of the lead frame of 3 layers subjected to roughening such as blast, and a production process of the same, a lead frame for lead-free soldering has a layer composition of a nickel plated film on a metal sheet surface, a palladium plated film on the nickel plate surface and a gold plated film on the surface thereof provided in order, a layer thickness of the nickel plated film is not uniform, each layer thickness of the palladium plated film and the gold plated film is uniformly formed, and the nickel plated film with the nonuniform layer thickness is subjected to wet blast on the surface and roughened to enhance the adhesion strength with the sealing resin.
Patent Document 9: Japanese Patent Application Laid-open No. 2007-115,925
According to the summary of patent document 9, it is disclosed that with an object of providing a plated lead frame of steadily wetting with lead-free solder even with a small outer lead pitch, a 4 layer plating of substrate plating (Ni) 10, palladium plating 11, silver plating 12 and gold plating 13 in order from the bottom layer is applied to a lead frame material 9.
Although gold-containing material plating having good connection stability has been used for a contact portion with an object, tin-containing material plating has come to be used as FIG. 7 in consideration of cost and environmental issues (it is possible to obtain enough connection stability not to cause a problem).
There has been a problem that with the tin-containing material plating, whiskers occur as FIG. 8 when an external force is added to a contact portion with an object where the external force should be added. If the whiskers occur, when the space between adjacent terminals is narrow, a short circuit occurs between the adjacent terminals, which causes poor connection.
Although multilayer surface treatment such as the patent documents 5 to 9 is disclosed, there is not one that a barrier intermediate layer for preventing the components of alloy and plating from diffusing into the top layer plating is provided. Moreover, there is not one that the whisker suppressing method cited in the patent documents 1 to 4 is conducted by the barrier intermediate layer for preventing the components of alloy and plating from diffusing into the top layer plating.
The inventors have analyzed a whisker portion in order to determine the cause of an occurrence of whiskers. A result is that nickel plating (approximately not less than 2 μm) as a substrate is applied to a material of copper alloy and a tin-containing material plating (approximately not less than 1.5 μm) is further applied thereto.
As seen from FIGS. 9 and 10, it is clear that the whisker portion contains copper and nickel as components other than tin. It can be thought that the growth of whiskers is furthered by diffusion of copper, which is a component in the copper alloy as the material, and nickel, which is a component in the nickel plating, into the tin-containing material plating. FIG. 9 (A) is a cross-sectional view (photograph) of a whisker occurred portion observed from a cross-sectional direction, (B) is an enlarged view of X portion of (A). The view (photograph) of FIG. 9 is a transmission electron image. An elemental analysis is conducted on points 1 and 2 covered with Y portion in FIG. 9 (B) by EDX (energy dispersive elemental X-ray spectroscopy). Point 1 of Y portion is a precipitate (inclusion) seen as black in the view and point 2 of Y portion is an area without precipitate. FIG. 10 (A) is an analysis result of point 1 and (B) is an analysis result of point 2. In FIG. 10 (A), copper (Cu) and nickel (Ni) are detected other than tin (Sn) as Z portion. However, nothing other than tin (Sn) is detected in point 2 as FIG. 10 (B). This shows that the above mentioned is clear.
Moreover, as shown in FIG. 11, a tin (Sn) crystal at the root in the whisker occurred portion is coarsened. This can be presumed that upon the application of external force to the tin-containing material layer, tin atoms tend to diffuse in a certain direction through an interfacial portion between the tin-containing material layer and a substrate layer and a tin plating grain boundary (grain boundary diffusion and intraparticle diffusion), since the diffusion of tin (Sn) atoms is concentrated on a certain tin (Sn) crystal particle, it is coarsened.
It can be thought that the 2 causes mentioned above (diffusion of copper (material) and nickel (substrate plating) into tin-containing material plating and coarsening of a certain crystal particle by diffusion of tin (Sn) atoms being concentrated thereon) correlatively further the occurrence of whiskers.